The present invention relates to an improved process for continuously crystallizing a solute or a mixture of solutes from a solvent or crystallizing a solvent from a solution, wherein the solution is brought to crystallization by cooling and/or evaporation. More particularly, the invention relates to an improved, continuous crystallization process in which the solution is brought to crystallization by cooling and/or evaporation in a first zone where crystal nuclei form and then, in a second zone, crystals grow to the desired dimensions.
A two stage crystallization has been described in British patent specification No. 977,686, published Dec. 9, 1964, in which partial crystallization of the solution is accomplished by cooling the solution in a scraped heat exchanger and the partially crystallized suspension is then introduced and continuously flows through a separate flowthrough device where the crystals are allowed to grow to the desired size. The suspension flow in the flowthrough device is plug flow with residence time being from 1 - 10 hours or longer. In such a process mixing of the entering suspension of small crystals and the suspension of larger crystals leaving the device is at most only negligible. The size and amount of crystals obtained in the plug flow through the crystallizer flowthrough device is comparable to the crystals obtained with a batchwise storage of the solution in a ripening vessel for the same period of time as the residence time in the flowthrough device.
An increase in residence time in the flowthrough device as described above produces a corresponding increase in the average size of the crystals produced. At a constant residence time, the size of crystals produced depends upon the size of the crystal nuclei in the solution coming from the scraped heat-exchanger, i.e., the larger the crystal nuclei produced in the scraped heat-exchanger, the larger the crystals will be which are produced in the flowthrough device. The average size of the crystals produced by the scraped heat-exchanger increases as the formation of actual nuclei in the heat-exchanger decreases. The formation of nuclei can be limited by keeping the heat flux, i.e., the removal of heat per unit time per unit surface of the heat-exchanger, at a small magnitude, as the small heat flux minimizes the undercooling of the suspension at the heat-extracting surface, thereby limiting the formation of crystal nuclei. The reduction in heat flux requires a large increase in cooling surface area and thus the equipment becomes large, bulky and expensive.
A seeding and filter technique for paraxylene production is disclosed in U.S. Pat. No. 2,757,216 dated July 31, 1956 in which paraxylene feedstock is first chilled to a temperature close to but just above the temperature at which paraxylene crystals form, then mixed with a slurry of crystals but at a temperature below the critical crystallization temperature. The chilled mixture is then fed into a holding tank and retained there for a period of from 30 minutes to 3 hours or longer, and larger seed crystals are introduced which also act as a filter aid. From the holding tank slurry is removed and a portion is admixed with the feedstock introduced into the chiller, the remainder sent to a basket centrifuge for crystal separation. The disclosure is quite specific in that a slurry of paraxylene crystals is recycled and introduced back into the chiller.
Crystal purification has been used in the past but has many disadvantages, especially in the food processing industry, as delicate flavor components may be lost by volatilization and oxidation of early-oxidized flavor components may lead to the production of unwanted off-flavors.
In general, in all prior art crystallization processes, the formation of crystal nuclei is controlled at a limited value so that large crystals are obtained rather than an increased number of small crystals. For example, in a crystallizer in which the crystallization is effected from a supersaturated solution, the formation of nuclei increases as the degree of supersaturation increases. As the number of crystal nuclei forming increases, the number of crystals produced increases; however, the average size of the crystals produced decreases. To limit the number of crystal nuclei being produced the heat flux is maintained at a low value which in turn requires a large amount of heat-exchanger surface. The equipment becomes bulky and expensive.